JP2022116956A - Electrode for reduction reaction - Google Patents

Abstract

To provide an electrode for reduction reaction capable of improving the selectivity of the synthesis of a multielectron reductant in a reduction reaction of carbon dioxide.SOLUTION: An electrode for a reduction reaction according to the present invention is used in a reduction reaction of carbon dioxide and comprises an electrode body containing an electrode catalyst and a non-conductive hydrophobic organic substance disposed on the electrode body to modify it.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a reduction reaction electrode.

To solve problems such as global warming and depletion of energy resources and carbon resources due to the increase in atmospheric carbon dioxide (CO ₂ ) concentration, research on reduction reaction electrodes used for reduction reactions of carbon compounds such as carbon dioxide is taking place around the world.

For example, Patent Document 1 discloses a conductive member and an adsorbent on the surface of the conductive member, the adsorbent having conductivity and pores and having a porous structure capable of adsorbing carbon dioxide. A carbon dioxide reduction electrode is disclosed which has a substrate and a metal complex having a central metal capable of reducing carbon dioxide in the pores.

Further, Patent Document 2 discloses a carbon dioxide reduction electrode having a metal-containing member capable of reducing carbon dioxide and an adsorbent capable of adsorbing carbon dioxide on the surface of the metal-containing member.

Further, Patent Document 3 discloses a reduction reaction electrode including an electrode modified with a silane coupling agent.

In addition, Non-Patent Document 1 discloses a carbon dioxide reduction technique using a metal electrode.

Non-Patent Document 2 discloses a copper metal electrode modified with a hydrophilic polymer.

Non-Patent Document 3 discloses a reduction reaction electrode in which graphite or carbon nanoparticles are laminated on the surface of a copper electrode formed on a PTFE filter.

JP 2017-125234 A JP 2017-78190 A JP 2017-101285 A

Yoshio Hori, "Electrolytic Reduction of Carbon Dioxide by Steel Electrode", Electrochemistry, 1990, 11, p996-1002 Aya K. Buckley et al. , J. Am. Chem. Soc. , 2019, 141, p7355-7364, "Electrocatalysis at Organic-Metal Interfaces: Identification of Structure-Reactivity Relationships for CO2 Reduction at Modified Cu Surfaces" Dinh, C.; -T et al. , Science 2018, 360, 783-787. "CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface"

By the way, in the reduction reaction of carbon dioxide, there is a demand for a reduction reaction electrode capable of efficiently producing a multi-electron reduced product with more than two electrons, which is more useful than a two-electron reduced product in terms of energy resources. The two-electron reduction products produced by the two-electron reduction of carbon dioxide are, for example, CO, HCOOH, and the like. Further, multi-electron reduction products generated by multi-electron reduction of carbon dioxide with more than two electrons include, for example, CH ₄ (8-electron reduction product), C ₂ H ₄ (12-electron reduction product), C ₂ H ₅ OH (12 electron reduced product) and the like.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a reduction reaction electrode capable of improving the selectivity of synthesis of a multi-electron reduced product in a carbon dioxide reduction reaction.

The present invention is a reduction reaction electrode used for a reduction reaction of carbon dioxide, characterized by comprising an electrode body containing an electrode catalyst and a non-conductive hydrophobic organic material modified on the electrode body. do.

Moreover, in the reduction reaction electrode, the electrode catalyst preferably contains a copper-based catalyst.

Moreover, in the reduction reaction electrode, the copper-based catalyst is preferably metallic copper.

Moreover, in the reduction reaction electrode, the non-conductive hydrophobic organic substance preferably contains triphenylphosphine.

Moreover, in the reduction reaction electrode, the non-conductive hydrophobic organic substance is preferably an aggregate of triphenylphosphine particles.

According to the present invention, it is possible to improve the selectivity of synthesizing a multi-electron reduced product in the reduction reaction of carbon dioxide.

1 is a schematic configuration diagram showing an example of a reduction reaction electrode according to an embodiment; FIG. 1 is a schematic configuration diagram showing an example of a carbon dioxide reduction device provided with a reduction reaction electrode according to the present embodiment; FIG. FIG. 2 is a schematic configuration diagram showing another example of a carbon dioxide reduction device provided with a reduction reaction electrode according to the present embodiment; FIG. 2 is a graph showing changes over time in the current efficiency of the product produced by the CO ₂ reduction reaction test of Example 1; FIG. 10 is a diagram showing changes over time in current efficiency of products produced by a CO ₂ reduction reaction test of a comparative example. FIG. 10 is a graph showing changes over time in the current efficiency of the product produced by the CO ₂ reduction reaction test of Example 2; FIG. 2 is a graph showing changes over time in cathodic current in CO ₂ reduction reactions of Examples 1 and 2 and Comparative Example. 1 is a schematic cross-sectional view of a reduction reaction electrode C of Example 1 for explaining a CO ₂ reduction reaction. FIG.

An example of the reduction reaction electrode according to this embodiment will be described below.

[Electrode for reduction reaction]
FIG. 1 is a schematic configuration diagram showing an example of a reduction reaction electrode according to this embodiment. The reduction reaction electrode 1 shown in FIG. 1 has an electrode body 10 and a modification layer 12 that modifies the surface of the electrode body 10 . In addition, in the reduction reaction electrode 1 shown in FIG.

Electrode body 10 has an electrode catalyst that is used to reduce carbon dioxide. Electrocatalysts include, for example, silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pd) , iron (Fe), and tantalum (Ta). Among these, for example, a copper-based catalyst containing a copper element is preferable in terms of improving the selectivity of synthesizing a multi-electron reduced product in the reduction reaction of carbon dioxide. Examples of copper-based catalysts include metallic copper, alloys of copper and other metals, copper complexes with copper as the central metal (eg, tetraphenylporphyrin Cu complexes), and the like. Among these, metallic copper is preferred. Further, the electrode body 10 may have a structure in which an electrode catalyst is supported on a substrate containing a carbon material, for example. Carbon materials include, for example, carbon nanotubes, graphene, carbon black, carbon cloth, carbon paper, glassy carbon, and graphite. In addition, it may contain a P-type semiconductor having a conductor bottom potential on the negative side of the reduction potential of carbon dioxide. Examples of P-type semiconductors include oxides such as silicon (Si), copper oxide (Cu ₂ O), zinc-doped iron oxide (Fe ₂ O ₃ ), indium phosphide (InP), gallium phosphide (GaP), and indium gallium. Phosphorus compounds such as phosphorus (InGaP), nitrogen compounds such as gallium nitride ₍ GaN) and _C3N4 , selenium compounds such as CIGS and CZTSSe, and the like.

Modification layer 12 includes a non-conductive hydrophobic organic material. A hydrophobic organic substance is an organic substance that is sparingly soluble or insoluble in water, and that has a solubility of less than 1 g in 100 g of pure water at 20°C. In addition, non-conductivity means a volume resistivity of 10 ¹³ Ωcm or more at 20°C. Hereinafter, non-conductive hydrophobic organic substances are simply referred to as hydrophobic organic substances.

Hydrophobic organic substances include, for example, trimethylphosphine, triethylphosphine, tri-n-propylphosphine, triisopropylphosphine, tri-n-butylphosphine, tri-tert. -butylphosphine, tri-n-pentylphosphine, tricyclopentylphosphine, tri-n-hexylphosphine, tricyclohexylphosphine, tri-n-heptylphosphine, tri-n-octylphosphine, triphenylphosphine, tris(2-methoxyphenyl ) phosphine, tris(4-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, tris(2-furyl)phosphine, tris(4-dimethylaminophenyl)phosphine, tri-p-tolylphosphine, tris- (4-fluorophenyl)phosphine, tris[3,5-bis(trifluoromethyl)phenyl]phosphine, tris(pentafluorophenyl)phosphine, bis(diphenylphosphino)methane, 1,2-bis(diphenylphosphino) Ethane, 1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,5-bis(diphenylphosphino)pentane, bis(dicyclohexylphosphinophenyl) ether, 1,1 '-Bis(dicyclohexylphosphino)ferrocene, 1,3-bis(diisopropylphosphino)propane, 1,4-bis(diisopropylphosphino)butane, 1,3-bis(dicyclohexylphosphino)propane, 1,4- Bis(dicyclohexylphosphino)butane, 1,2-bis(diphenylphosphino)benzene, 2,2′-bis(diphenylphosphino)biphenyl, 4,6-bis(diphenylphosphino)phenoxazine, bis(dimethylphosphino) phino)methane, 1,2-bis(dimethylphosphino)ethane, 1,3-bis(dimethylphosphino)propane, 1,4-bis(dimethylphosphino)butane, 1,5-bis(dimethylphosphino) Organophosphorus compounds such as pentane, 1,6-bis(dimethylphosphino)hexane, 1,2'-bis(dimethylphosphino)benzene, and 2,2'-bis(dimethylphosphino)biphenyl can be mentioned. In addition, for example, polystyrene (PS), polymethyl methacrylate (PMMA), polycarbonate (PC), polypropylene (PP), polyethylene terephthalate (PET), polyethylene (PE), polyvinylidene fluoride (PVDF), polymethylpentene ( PMP), polylactic acid (PLA), cyclic olefin copolymers (COC) and cyclic olefin polymers (COP). Among these, for example, triphenylphosphine is preferable from the viewpoint of improving the selectivity of C2 compound synthesis among multi-electron reductant synthesis in the reduction reaction of carbon dioxide.

The modification layer 12 is formed, for example, by coating the electrode body 10 with a solution in which a hydrophobic organic substance is dissolved in an organic solvent, or by immersing the electrode body 10 in the solution. By such a method, for example, a reduction reaction electrode having a sea-island structure having a region (island) where the modified hydrophobic organic substance aggregates on the electrode body 10 and a region (sea) where the electrode body 10 is exposed can be obtained. . Any organic solvent can be used as long as it dissolves hydrophobic organic substances, and examples thereof include tetrahydrofuran, acetone, 2-propanol, ethanol, and acetonitrile.

The modification layer 12 is formed by coating the electrode body 10 with a liquid obtained by developing a solution of a hydrophobic organic substance dissolved in an organic solvent on water such as pure water or ultrapure water, or applying the liquid to the electrode body 10 . It may be formed by immersing it in the liquid. By preparing a liquid in which a solution in which a hydrophobic organic substance is dissolved in an organic solvent is developed on water, the hydrophobic organic substance self-assembles, so that the electrode body 10 is coated with or immersed in the liquid. As a result, agglomerates of hydrophobic organic particles (for example, particle diameters of several μm) can be formed on the electrode body 10 . In other words, a reduction reaction electrode in which aggregates of hydrophobic organic particles are dispersed on the electrode body 10 is obtained. A liquid obtained by developing a solution of a hydrophobic organic substance dissolved in an organic solvent on water may be used immediately after preparation, or may be used after being left for several hours to several days. Further, a suspension obtained by developing a solution in which a hydrophobic organic substance is dissolved in an organic solvent in water is filtered through a filtration membrane, and aggregates of hydrophobic organic particles obtained as a filtrate on the filtration membrane are applied to an electrode. It may be applied to body 10 .

From the viewpoint of reduction reactivity of carbon dioxide, it is preferable that the above-described sea-island structure and aggregates of hydrophobic organic particles are dispersed.

The contact member 14 is not particularly limited as long as it can connect the conductor 16 to the electrode body 10, and examples thereof include copper tape, indium, carbon tape, lead, tin, and tantalum.

The conducting wire 16 is not particularly limited as long as it is an electrically conductive member, and examples thereof include a copper wire and an aluminum wire.

When a voltage is applied to the reduction reaction electrode 1, electrons generated in the electrode body 10 are used for the reduction reaction of carbon dioxide. By the reduction reaction of carbon dioxide, for example, two-electron reduced products such as CO and HCOOH, CH ₄ (8-electron reduced product), C ₂ H ₄ (12-electron reduced product), C ₂ H ₅ OH (12-electron reduced product) A multi-electron reduction product with more than two electrons such as is generated. Here, in the present embodiment, the hydrophobic organic material modified on the electrode body 10 suppresses the diffusion of the reaction intermediates generated by the reduction of carbon dioxide, and the reaction intermediates can be stagnant around the electrode body 10. Therefore, it is considered that the reaction intermediate is easily reduced to a multi-electron reduction product. Therefore, the reduction reaction electrode 1 of the present embodiment can improve the selectivity of multi-electron reduction product synthesis compared to a reduction reaction electrode in which the electrode body is not modified with a hydrophobic organic substance.

Further, according to the reduction reaction electrode 1 of the present embodiment, the hydrophobic organic substance modified on the electrode body 10 suppresses contact of water molecules and protons with the surface of the electrode body 10 . As a result, the reduction reaction electrode 1 of the present embodiment can suppress the hydrogen generation reaction, which is a side reaction, compared to a reduction reaction electrode in which the electrode body is not modified with a hydrophobic organic substance.

In the reduction reaction electrode 1 of the present embodiment, the electrode Preferably, the body 10 is modified with aggregates of particles such as triphenylphosphine.

In the reduction reaction electrode 1, the water contact angle of the electrode body 10 modified with a hydrophobic organic substance is such that it suppresses the production rate of hydrogen due to side reactions, increases the production rate of reduced products due to the reduction reaction of carbon dioxide, and the like. , the angle is preferably 89 degrees or more, and more preferably 95 degrees or more.

The water contact angle was obtained by dropping 0.5 μL of water droplets onto the sample surface (electrode surface modified with a hydrophobic polymer) using a contact angle meter, and photographing the shape of the water droplets immediately after dropping. It is obtained by measuring from the image using the θ/2 method. Here, for example, in the case of a porous electrode body in which an electrode catalyst is supported on a base material such as carbon cloth or carbon paper, even if water droplets are dropped on the surface of the porous electrode body modified with a hydrophobic organic substance, the water droplets permeates inside and the water contact angle cannot be measured. In this case, an electrode catalyst is supported on a smooth substrate (for example, a glass substrate) under the same conditions, and a hydrophobic organic substance is modified on the electrode catalyst under the same conditions as a sample. Let the measured water contact angle be the water contact angle of the electrode body surface modified with the hydrophobic organic substance.

"Carbon compound reduction device"
FIG. 2 is a schematic configuration diagram showing an example of a carbon dioxide reduction device provided with a reduction reaction electrode according to this embodiment. The carbon dioxide reduction device 3 shown in FIG. 2 includes a first electrode (cathode) 1 which is the electrode for reduction reaction described above, and an electrode for oxidation reaction which is electrically connected to the first electrode 1 and which causes an oxidation reaction. and an electrolyte solution including a cathode compartment electrolyte solution 30 and an anode compartment electrolyte solution 32 .

In the carbon dioxide reduction apparatus 3 shown in FIG. 2, for example, the inside of the container 26 is separated into the cathode chamber 22 and the anode chamber 24 by the diaphragm 28, and the cathode chamber 22 contains the cathode chamber electrolyte solution 30, and the anode chamber 24 contains an anode compartment electrolyte solution 32 . The first electrode 1, which is an electrode for reduction reaction, is immersed in the cathode chamber electrolyte solution 30, and the second electrode 20 is immersed in the anode chamber electrolyte solution 32. As shown in FIG. When performing the carbon dioxide reduction reaction, carbon dioxide (CO ₂ ) is supplied to the cathode chamber electrolyte solution 30 so that the cathode chamber electrolyte solution 30 contains carbon dioxide.

FIG. 3 is a schematic configuration diagram showing another example of a carbon dioxide reduction apparatus provided with a reduction reaction electrode according to this embodiment. In the carbon dioxide reduction device 4 shown in FIG. 3, the same components as those of the carbon dioxide reduction device 3 shown in FIG. 2 are denoted by the same reference numerals. A reference electrode 48 is installed in the cathode chamber 22 in the carbon dioxide reduction device 4 shown in FIG. Further, the first electrode (cathode) 1, which is the electrode for the reduction reaction described above, is fitted in the side wall of the container 26 on the cathode chamber 22 side. However, in the first electrode 1 , the surface modified with the hydrophobic organic substance is exposed from the side wall of the container 26 and is in contact with the cathode chamber electrolyte solution 30 . The second electrode 20 is fitted in the side wall of the container 26 on the anode chamber 24 side, and one main surface of the second electrode 20 is exposed from the side wall of the container 26 and is in contact with the anode chamber electrolyte solution 32 . . Although not shown, a plurality of through channels for supplying carbon dioxide (CO ₂ ) from the outside to the first electrode 1 are provided in the side wall of the container 26 in which the first electrode 1 is fitted. . That is, when performing the carbon dioxide reduction reaction, carbon dioxide is supplied from the outside of the container 26 to the first electrode 1 through the through channel provided on the side wall of the container 26, and flows through the first electrode 1. It diffuses and blows into the cathode compartment electrolyte solution 30 in the cathode compartment 22 .

In the carbon dioxide reduction device (3, 4) of the present embodiment, the first electrode 1 and the second electrode 20 are electrically connected, and by applying an appropriate bias voltage, the second electrode At 20, an oxidation reaction takes place and a potential is obtained. At the first electrode 1, the reduction reaction of carbon dioxide proceeds by obtaining a potential from the electrode that causes the oxidation reaction.

Means for applying a bias voltage are not particularly limited, and examples thereof include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, solar batteries, and the like.

Electric power supplied to the carbon dioxide reduction device (3, 4) and the first electrode 1 and the second electrode 20 shown in FIGS. and a photovoltaic cell. In the artificial photosynthesis apparatus according to this embodiment, the first electrode 1 and the second electrode 20 of the carbon dioxide reduction apparatus (3, 4) are connected via a solar cell and driven using sunlight as an energy source.

Cathode compartment electrolyte solution 30 includes a cathode compartment electrolyte. Potassium chloride (KCl), potassium hydrogen carbonate (KHCO ₃ ), potassium sulfate (K ₂ SO ₄ ), potassium carbonate (K ₂ CO ₃ ), potassium tetraborate (K ₂ B ₄ O ₇ ), dipotassium hydrogen phosphate ₍ K ₂ HPO ₄ ), potassium dihydrogen phosphate (KH ₂ PO ₄ ), and the like. From the point of view, potassium chloride (KCl), potassium hydrogen carbonate (KHCO ₃ ), and the like are preferable.

The anode chamber electrolyte solution 32 may be pure water, water vapor, or the like when a cation exchange membrane is used as the diaphragm. The anode compartment electrolyte solution 32 may include an anode compartment electrolyte. Electrolytes for the anode chamber include sodium hydrogen carbonate (NaHCO ₃ ), potassium hydrogen carbonate (KHCO ₃ ), potassium carbonate (K ₂ CO ₃ ), potassium sulfate (K ₂ SO ₄ ), potassium tetraborate (K ₂ B ₄ O ₇ ), dipotassium hydrogen phosphate (K ₂ HPO ₄ ), potassium dihydrogen phosphate (KH ₂ PO ₄ ), and potassium hydroxide (KOH). Potassium hydrogen carbonate (KHCO ₃ ), potassium tetraborate (K ₂ B ₄ O ₇ ), potassium hydroxide (KOH), and the like are preferable because O ₂ generation due to the oxidation reaction of is facilitated.

As a solvent for the cathode chamber electrolyte solution 30 and the anode chamber electrolyte solution 32, water may be used.

The electrolyte concentration of the cathode chamber electrolyte solution 30 is, for example, in the range of 0.01 mol/L to 8 mol/L, preferably in the range of 0.1 mol/L to 3.0 mol/L.

The electrolyte concentration of the anode chamber electrolyte solution 32 is, for example, in the range of 0.01 mol/L to 8 mol/L, preferably in the range of 0.1 mol/L to 3.0 mol/L.

The second electrode 20 is an electrode that is used to oxidize substances through an oxidation reaction. As the second electrode 20, platinum, gold, carbon, mercury, fluorine-containing tin oxide (FTO), tin-doped indium oxide (ITO), iridium oxide (IrOx: x=1 to 2) or a cobalt compound on a semiconductor substrate. A modified electrode, a composite material containing nickel or nickel and iron, or the like can be used. Semiconductor substrates include tungsten oxide (WO ₃ ), bismuth vanadate (BiVO ₄ ), iron oxide (Fe ₂ O ₃ ), silicon (Si), tantalum oxynitride (TaON), and the like.

Examples of the diaphragm 28 include proton exchange membranes (cation exchange membranes), anion exchange membranes, bipolar membranes, porous glass membranes and the like, and anion exchange membranes and the like can be preferably used. Although it is preferable to provide the diaphragm 28 in terms of efficient progress of the reduction reaction of the carbon compound at the first electrode 1 (reduction reaction electrode), the diaphragm 28 may not be provided.

As the storage container 26, for example, a closed container made of metal, plastic, glass, or the like, a reaction container having a mechanism for circulating gas, or the like can be used.

The carbon dioxide reduction device 3 shown in FIG. 2 is of a two-electrode type using a reduction reaction electrode and an oxidation reaction electrode, but is not limited to this, and may be of a three-electrode type combining a reference electrode. The reference electrode is installed, for example, on the cathode chamber 22 side, similarly to the carbon dioxide reduction device 4 shown in FIG.

EXAMPLES The present invention will be further described below with reference to Examples, but the present invention is not limited to these Examples.

<Electrode A for reduction reaction>
A film of copper was formed on a water-repellent carbon paper (TGP-H-060-H manufactured by Chemics Co., Ltd.) to prepare an electrode body. This electrode body was used as electrode A for reduction reaction. An RF magnetron sputtering method was used to deposit copper on the carbon paper. Specifically, in a sputtering apparatus with a movable mask mechanism (Canon Tokki SPK-404L), under the conditions of an argon (Ar) gas flow rate of 50 sccm and a pressure of 0.5 Pa, a copper target is discharged at an output power of 200 W, and carbon paper A film having a thickness of about 100 nm was formed thereon.

<Reduction reaction electrode B>
The reduction reaction electrode A (electrode body) was immersed in 1 ml of a solution in which 1 mmol of triphenylphosphine ₍ PPh ₃ ) was dissolved in 50 ml of acetone, and dried under reduced pressure. Electrode B was produced.

<Electrode C for reduction reaction>
A solution of 0.033 mmol of triphenylphosphine (PPh ₃ ) dissolved in 10 ml of acetonitrile was put into 20 ml of ultrapure water (Milli-Q), and the solution changed to a white suspension was passed through a membrane filter (PTFE, pore size 0.45 μm). ) was used to perform suction filtration. After suction filtration, the filtrate composed of PPh3 particles _on the membrane filter was collected. The collected PPh ₃ particles were applied to the reduction reaction electrode A (electrode body) and then dried to prepare a reduction reaction electrode C in which the electrode body was modified with aggregates of PPh ₃ particles.

< _CO2 reduction reaction test>
(Example 1)
A _CO2 reduction reaction test was performed using the carbon dioxide reduction apparatus shown in FIG. In Example 1, the reduction reaction electrode C was used as the first electrode 1, which is the working electrode, and platinum foil (PT-353212, manufactured by Nilaco Corporation, φ20 mm × 0.02 mm, 99.98%) was used as the second electrode 20, which was the counter electrode. , an Ag/AgCl electrode (manufactured by EC Frontier, RE-14) was used as the reference electrode 48 . An anion exchange membrane (manufactured by Astom, ASE) was used as the diaphragm 28 . The storage container 26 is separated into a cathode chamber 22 and an anode chamber 24 by a diaphragm 28, and 0.5 mol/L hydrogen carbonate is used as a cathode chamber electrolyte solution 30 in the cathode chamber 22 and an anode chamber electrolyte solution 32 in the anode chamber 24. A potassium aqueous solution was used.

The first electrode 1, the second electrode 20 and the reference electrode 48 are connected to an electrochemical measurement system (Bio-Logic Science Instruments, SP-150), CO ₂ gas (99.995%) at a flow rate of 10 ml / min, While supplying the cathode chamber electrolyte solution 30 in the cathode chamber 22 from the outside through the through channel provided on the side wall of the container 26 and the first electrode 1, the first electrode 1 is supplied with −2.0 V vs Ag/AgCl. A _CO2 reduction reaction test was performed by applying voltage for 2 hours. An on-line gas chromatograph (SRI Instruments Multiple Gsa Analyzer #5) was used for product identification and quantification associated with the _CO2 reduction reaction test. MOLECULAR SIEVE, SA and HAYESEP-D were used as columns, and thermal conductivity detectors (TCD) and flame ionization detectors (FID) were used as detectors.

(Example 2)
Except that the reduction reaction electrode B was used as the first electrode 1 instead of the reduction reaction electrode C, and that a voltage of −2.0 V vs Ag/AgCl was applied to the first electrode 1 for 3 hours. Measurement was performed in the same manner as in 1.

(Comparative example)
Measurement was performed in the same manner as in Example 1, except that the reduction reaction electrode A was used as the first electrode 1 instead of the reduction reaction electrode C.

FIG. 4 shows the time course of the current efficiency of the product produced by the _CO2 reduction reaction test of Example 1, and FIG. 5 shows the time course of the current efficiency of the product produced by the _CO2 reduction reaction test of the comparative example. , and FIG. 6 shows the time course of the current efficiency of the product produced by the CO ₂ reduction reaction test of Example 2. Table 1 also shows the current efficiency of the product at the end of the CO ₂ reduction reaction test of Examples 1 and 2 and Comparative Example. The current efficiencies in FIGS. 4 to 6 show instantaneous values in measurements every 20 minutes, and the current efficiencies in Table 1 show the overall values (average values) over the measurement time (2 hours or 3 hours). .

As shown in Table 1, in Example ₁ , the current efficiencies of _C2H4 and _C2H5OH , which are _C2 compounds and are 12-electron reduction products of _CO2 , are 29.5% and 10.2%, respectively. Met. Also, the current efficiency of CH4, which is an ₈ -electron reduction product of _CO2 , was 14.1%. On the other hand, the current efficiencies of CO and HCOOH, _two -electron reduction products of _CO2 , were both 5.6%, and the current efficiency of H2, a by-product, was 31.0%.

In addition, as shown in Table 1, the comparative example is a _C2 compound, and the current efficiencies of _C2H4 and _C2H5OH , which are ₁₂ -electron reduction products of _CO2 , are 10.1% and 3.2%, respectively. %Met. On the other hand, the current efficiencies of CO and HCOOH, which are _two -electron reduction products of CO2, are 8.0% and 9.2%, respectively, and the current efficiency of H2, a by _- product, is 48.7%. there were.

From these results, Example 1 using a reduction reaction electrode in which aggregates of PPh ₃ particles were modified on the electrode body was compared with an electrode body in which a copper film was formed on carbon paper as a reduction reaction electrode. From the examples, it can be said that the selectivity of the multi-electron reductant synthesis was improved. More specifically, it can be said that the selectivity of the C2 compound synthesis among the multi-electron reduction product syntheses was improved. In addition, in Example 1, the production reaction of H ₂ as a by-product was suppressed more than in Comparative Example.

Further, as shown in Table 1, in Example ₂ , the current efficiencies of _C2H4 and _C2H5OH , which are C2 compounds and are 12-electron reduction products of _CO2 , are 11.5% and _3.5 %, respectively. was 8%. Also, the current efficiency of CH4, which is an ₈ -electron reduction product of _CO2 , was 20.5%. _On the other hand, the current efficiency of H2, a by-product, was 42.9%.

From these results, Example 2, in which the electrode body was immersed in a solution containing PPh ₃ and the electrode body for reduction reaction was modified with PPh ₃ on the electrode body, was an electrode body in which a copper film was formed on carbon paper. It can be said that the selectivity in synthesizing a multi-electron reduced product was improved as compared with the comparative example used as a reduction reaction electrode. Moreover, in Example 2, the production reaction of H ₂ as a by-product was suppressed more than in Comparative Example. Comparing Example 1 and Example 2, Example ₁ using a reduction reaction electrode in which an aggregate of PPh3 particles was modified _on the electrode body was immersed in a solution containing PPh3. , the production reaction of H ₂ was suppressed and the selectivity for synthesizing a C2 compound among multi-electron reductant syntheses was improved, as compared with Example 2 using a reduction reaction electrode B in which PPh ₃ was modified on the electrode body.

FIG. 7 shows changes over time in cathodic current in the CO ₂ reduction reactions of Examples 1 and 2 and Comparative Example. In addition, Table 2 shows the amount of products produced per unit time (μmol/h) in the CO ₂ reduction reaction of Examples 1 and 2 and Comparative Example.

As shown in FIG. 7, the comparative example has a higher cathode current value than the second example. Therefore, in Comparative Example, the current efficiency of the multi-electron reduced product is lower than that of Example 2, but as shown in Table 2, the amount of the multi-electron reduced product produced per unit time is increased. On the other hand, the cathodic current value of Example 1 is about the same as that of Example 2, but the amount of multi-electron reduction product produced per unit time, specifically the amount of _C2 compound produced, is higher than that of Comparative Example. The production amount is lower than the comparative example. These results suggest that the modified forms of PPh3 that _are modified on the electrode body also affect the selectivity of multi-electron reductant synthesis.

FIG. 8 is a schematic cross-sectional view of the reduction reaction electrode C of Example 1 for explaining the CO ₂ reduction reaction. In addition, the broken-line frame in the reduction reaction electrode C of Example 1 of FIG. 8 is shown by the enlarged view of the said part. As shown in FIG. 8, in the reduction reaction electrode C of Example 1, protons and the like in the electrolyte solution diffuse to the surface of the electrode body 10 due to the aggregates 12a of the PPh3 particles modified _on the surface of the electrode body 10. is inhibited, the _H2 production reaction is suppressed. In addition, the aggregate 12a of the PPh3 particles modified _on the surface of the electrode body 10 suppresses the diffusion of the reaction intermediates of the CO ₂ reduced product, and the reaction intermediates stay around the electrode body 10. It is thought that the electron reduction product (particularly, the C2 compound) is easily reduced. In addition, in the reduction reaction electrode A of the comparative example, the protons in the electrolyte solution move to the electrode body without hindrance, so that the H ₂ production reaction is promoted. In addition, since the CO ₂ reduction product generated on the surface of the electrode body does not stay around the electrode body and separates from the electrode body, it is difficult to be reduced to the multi-electron reduction product.

1 reduction reaction electrode 3,4 carbon dioxide reduction device 10 electrode body 12 modification layer 12a aggregate of PPh ₃ particles 14 contact member 16 conducting wire 22 cathode chamber 24 anode chamber 26 container 28 Diaphragm, 30 cathodic chamber electrolyte solution, 32 anodic chamber electrolyte solution, 48 reference electrode.

Claims (5)

A reduction reaction electrode used for a reduction reaction of carbon dioxide,
A reduction reaction electrode comprising: an electrode body containing an electrode catalyst; and a non-conductive hydrophobic organic substance modified on the electrode body. 2. The reduction reaction electrode according to claim 1, wherein the electrode catalyst contains a copper-based catalyst. 3. The reduction reaction electrode according to claim 2, wherein the copper-based catalyst is metallic copper. 4. The reduction reaction electrode according to claim 1, wherein the non-conductive hydrophobic organic substance contains triphenylphosphine. 4. The reduction reaction electrode according to claim 1, wherein the non-conductive hydrophobic organic substance is an aggregate of triphenylphosphine particles.

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